I understand gravitational differentiation caused the layered structure of Earth when it is still molten. However, why are the heavier constituents in a reduced state, i.e. metal $\text{Fe}$ and $\text{Ni}$, instead of an oxidized state? Is this because of the high temperature and pressure?

  • $\begingroup$ What James said. My guess is the high temp, but I had no luck searching for info about the thermal decomposition temperature of iron oxides to metallic iron. $\endgroup$
    – PM 2Ring
    Mar 24, 2019 at 8:30
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    $\begingroup$ Maybe because there is no oxygen there? $\endgroup$
    – peterh
    Mar 24, 2019 at 10:59
  • $\begingroup$ Or probably because Fe and Ni are heavier than O. However, this questions should be migrated to Earth Science SE. $\endgroup$
    – Pere
    Mar 24, 2019 at 14:59
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    $\begingroup$ I think the answer is that matter is distributed to minimise the gravitational potential energy, taking into account the densities of oxides vs reduced forms and the chemical reaction energies. There is not enough oxygen to oxidise all the iron, silicon and magnesium, so the lowest energy is achieved with reduced iron at the core. This is planetary science question, but the expertise may be in chemistry.SE . $\endgroup$ Mar 24, 2019 at 17:37
  • $\begingroup$ It's thought that Iron-Nickel cores don't form until a certain temperature and liquid core is reached. But whether that's because there's pure Iron and Nickel to begin with or whether Oxygen is worked out of it . . . I'm not sure. Some pure iron and nickel likely exist in rocky meteors, just spread out, not concentrated in the center. en.wikipedia.org/wiki/Meteorite#Meteorite_types That said, as it has to do planetary formation, so I think it fits under astronomy as well as earth science. $\endgroup$
    – userLTK
    Mar 29, 2019 at 1:26

2 Answers 2


Because there wasn't enough oxygen to oxidise all of it. There is only so much oxygen on Earth. Most of it went to oxidise the elements that have higher affinities to oxygen: silicon, magnesium, calcium, aluminium, etc. Iron and nickel have lesser affinity to oxygen, so you start by oxidising some of it, until there is no oxygen left.

But there is still unoxidised iron and nickel left! So this just sinks to the core.

Further reading.


Because the early earth and the current surface of the earth are very different conditions.

Any oxidized iron present on the forming earth would have quickly ceased to be oxidized as the planet became molten, Once heated the oxygen would have quickly been stripped away by elements with higher affinity. Silicon for instance has a much higher affinity for oxygen, hence why much of the planet is composed of silicates. During formation as the the earths material separated out by density. oxygen and oxides are just too light compared to heavier metals. Any oxides present at the very beginning would have quickly given up the oxygen to other materials with higher affinity once it was heated as the earth built up enough mass and energy to become molten.

Today it is hard to find unoxidized iron on the surface, but life had to lock up many of the other oxidizable materials before iron started to oxidize in large amounts. Most of the oxidized iron on earth was only oxidized after photosynthesis evolved and flooded the atmosphere with oxygen (from splitting water), the core is incredibly well insulated from that oxygen, free oxygen is too light to be transported down by mantle convection. Not that it would have made much difference even if you used all the oxygen in the atmosphere it would not oxidize even 1% of the core.

  • $\begingroup$ This answer is incorrect. The oxygen generated by photosynthesis didn't appear out of nowhere. This was oxygen that was already present on Earth in various forms, and would have participated in the core–mantle differentiation process. $\endgroup$
    – Gimelist
    Mar 30, 2019 at 9:01
  • $\begingroup$ To add some more: surface oxidation doesn't go deep. Maybe 100 metres deep in supergene environments. If you have some subducted sediments in a downgoing slab you may be lucky if the oxidised stuff reaches tens or several hundreds of kilometres. The core is at a depth of about 3000 kilometres. There is no influence whatsoever of any atmospheric oxidation on something this deep. $\endgroup$
    – Gimelist
    Mar 30, 2019 at 9:10
  • $\begingroup$ @Gimelist it was present in the form of water, hydrogen has a much higher affinity than iron, so that oxygen really was not available to oxidize the iron. hence the term "free oxygen" $\endgroup$
    – John
    Mar 30, 2019 at 13:39
  • $\begingroup$ and CO2. But that’s not the point. Your answer implies that if atmospheric oxygen did exist, then it could have oxidise the iron. No, it would have oxidised whatever it was made from in the first place (be it hydrogen and carbon). Those are two different separate processes, or “cycles”. Furthermore, even if we had 100% O2 in the atmosphere it wouldn’t even scratch the surface of the core. The mass of the core is a completely different order of magnitude to the mass of the atmosphere. $\endgroup$
    – Gimelist
    Mar 30, 2019 at 19:22
  • $\begingroup$ except that hydrogen is locked in a less accessible form, that is why the oxygen oxidized so much iron during the great oxygenation event. free oxygen could build up in the atmosphere until after it had oxidized all the available iron. yes the amount of the core it could oxidize would be insignificant but that is the only source ot oxidize any of it $\endgroup$
    – John
    Mar 31, 2019 at 5:26

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